As an emission type electronic display device, an electroluminescent device (ELD) is known. Elements constituting the ELD include an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to also as an organic EL element). Inorganic electroluminescent element has been used for a plane light source, however, a high voltage alternating current has been required to drive the element. An organic EL element has a structure in which a light emitting layer containing a light emitting compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emitting layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage, namely, several volts to several tens of volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, therefore, the element is noted from the viewpoint of space saving and portability.
For the practical use in the future, an organic EL element is desired to emit light of high luminance with high efficiency at a lower power.
For example, disclosed is an organic EL element exhibiting higher luminance of emitting light with longer life in which a stilbene derivative, a distyrylarylene derivative or a tristyrylarylene derivative doped with a slight amount of a fluorescent compound is employed (refer to Japanese Patent No. 3093796).
Also known are: an organic EL element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (for example, refer to Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-264692); and an organic EL element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (for example, refer to JP-A No. 3-255190).
When light emitted through excited singlet state is used in the organic EL element as disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, because the generation probability of excited species capable of emitting light is 25%, since the generation ratio of singlet excited species to triplet excited species is 1:3, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, has been reported by Prinston University (refer to M. A. Baldo et al., nature, 395, 151-154 (1998)), studies on materials emitting phosphorescence at room temperature have been actively carried out.
Examples are also reported in M. A. Baldo et al., Nature, 403(17), 750-753 (2000) or in U.S. Pat. No. 6,097,147.
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times higher than that of the excited singlet. Accordingly, light emission employing the excited triplet may enable almost the same performance as a cold cathode tube, and it is attracting attention to be applied as an illuminator.
For example, S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001) reports that many kinds of heavy metal complexes such as iridium complexes have been synthesized and studied.
In above mentioned M. A. Baldo et al., Nature, 403(17), 750-753 (2000), an example employing tris(2-phenylpyridine)iridium as a dopant has been studied.
As other examples, M. E. Tompson et al. have reported the application of L2Ir(acac) such as (ppy)2Ir(acac) as a dopant in the 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu), and Moon-Jae Youn. 0g, Tetsuo Tsutsui et al., have reported the application of tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3), and tris(benzo[h]quinoline)iridium (Ir(bzq)3) as a dopant in the 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu). These metal complexes are generally referred to as an ortho metalated iridium complex.
Also in aforementioned S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001), an application of various iridium complexes to an organic EL elements has been examined.
In order to obtain a higher emission efficiency, Ikai et al. have reported an application of a hole transport compound as a host material of a phosphorescent compound in the 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu). Also, M. E. Tompson et al., have reported an application of various electron-transport compounds as a host material of a phosphorescent compound, which is further doped with a novel iridium complex.
An ortho metalated complex having platinum as a central metal instead of iridium is also attracting attention. Many examples of this type of complex having a characteristic ligand have been known (for example, refer to Patent Documents 1-5).
Since each of the above examples is related to phosphorescent emission, the luminance, and the emission efficiency are notably improved compared to the conventional organic EL elements, however, the emission life of each element have been shorter than those of the conventional organic EL elements. It has not been fully easy to overcome the problem of a high emission efficiency phosphorescent material that the emission wavelength tends to shift to a shorter wavelength range and that the emission life of the organic EL element is not fully long, and a fully satisfactory performance for the practical use has not been obtained.
As a material to improve the performance, known is an iridium complex or a platinum complex each having a phenyl imidazole derivative as a ligand (for example, refer to Patent Documents 6-8). However, the emission efficiencies exhibited by these complexes are not fully satisfactory, and a further improvement has been desired.    Patent Document 1 Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2002-332291    Patent Document 2 JP-A No. 2002-332292    Patent Document 3 JP-A No. 2002-338588    Patent Document 4 JP-A No. 2002-226495    Patent Document 5 JP-A No. 2002-234894    Patent Document 6 WO 02/15645    Patent Document 7 WO 05/7767    Patent Document 8 JP-A No. 2005-68110